This invention relates broadly to fluid flow detection and, more particularly, pertains to air flow detection so as to control the operation of an accessory item in a blower-driven heating, ventilating or air conditioning (HVAC) system.
Operation of duct-mounted HVAC accessories, such as electronic air cleaners, humidifiers, dehumidifiers and UV light units, is often dictated by whether the HVAC system blower is running. The assignee of this application has traditionally employed their Model 50/51 Relay for detecting the status of the HVAC blower. The Model 50/51 Relay is a combination transformer and relay that activates when it senses current to the blower motor. The Model 50/51 Relay interfaces by way of a two-wire design to the HVAC blower by clamping around the return lead of the blower motor. The two wire interface allows a current to flow to the accessory item whenever there is current to the blower motor. Although this device has been satisfactory for powering many accessory items when the blower operates, it requires additional wiring to an external device that interfaces in the HVAC blower.
An alternative method of detecting when the blower is operating involves sensing of the actual air flow in the HVAC duct in a manner which eliminates the aforementioned external wiring. However, such method must be capable of detecting air flow over a wide range of and at extreme temperatures.
One method of interest is the dissipation contrast method. This method relies on the principle that heat dissipation is directly related to the speed of air flowing over a heat source. This method most commonly uses a pair of temperature sensing elements that are each exposed to a heat source. One element is in the air stream while the other, acting as a reference, is hidden from the air stream. If the sensors have the same response characteristics, the speed of the air stream can be derived from the difference in output from each of the sensing elements. Some examples of methods using variations of this embodiment are summarized below. Each of the following are well known methods of either sensing the presence of air flow or actually measuring the speed of the air stream;
The thermal or hot wire anemometer consists of a heated RTD, thermocouple junction, or thermistor sensor constructed at the end of a probe; it is designed to provide a direct, simple method of determining air velocity at a point in the flow field. The probe is placed into an airstream and the movement of air past the electrically heated velocity sensor tends to cool the sensor in proportion to the speed of the air flow. Often the sensor probe also incorporates an ambient temperature-sensing RTD or thermistor, in which the indicated air velocity is xe2x80x9ctemperature compensatedxe2x80x9d to xe2x80x9cstandardxe2x80x9d air density conditions (typically 0.0748 pounds per cubic foot).
Scientific Technologies employs two methods of calorimetric thermal dispersion mass flow monitoring. In the constant power method, the calorific power of the body is kept constant. The volume flow is determined by measuring the temperature difference between two sensors. In the constant temperature difference method, the temperature difference between two sensors is kept constant. The volume flow is determined by measuring the calorific power required to maintain the temperature difference.
Texas Instruments"" Klixon 2 SE Solid State Vane Switch uses a positive temperature coefficient (PTC) temperature sensor to provide air flow sensing. The PTC sensor remains at a low, relatively constant level of resistance over a wide temperature range then abruptly increases resistance at an elevated temperature known as the anomaly temperature. As the transition is approached, a slight temperature rise causes a dramatic increase in resistance. The TI sensor uses the PTC sensor for air flow detection in the following manner. Power is supplied to the PTC to cause it to self heat to a high resistance condition. Sufficient air flow will cool the sensor to its low resistance level. Insufficient air flow allows the sensor to self heat and reach a high resistance state. This resistance change and accompanying change in current is used to trigger an output corresponding to air flow present or not present.
U.S. Pat. No. 4,686,450.
U.S. Pat. No. 4,686,450 discloses an embodiment in which two sensor elements are used to detect air flow by generating a differential output that can be related to the speed of the airstream. The unique feature of this embodiment is a voltage reference that varies predictably with temperature to provide temperature compensation.
U.S. Pat. No. 5,710,380.
U.S. Pat. No. 5,710,380 discloses an embodiment in which a sensor element is heated to an elevated temperature above ambient. Once the elevated temperature is reached, the heat source is terminated and a sensor element is expected to cool. The rate of cooling is proportional to the speed of the air stream.
U.S. Pat. No. 4,733,559.
U.S. Pat. No. 4,733,559 discloses an embodiment in which two sensor elements are arranged in proximity to a heat source. The arrangement is such that one sensor will be upstream of the heater and the other will be downstream. When there is no air flow, the sensor outputs will be balanced. It is expected that an output differential will exist between the two sensing elements when air flow is present. The differential is a result of the upstream sensor being cooled by the air stream while the downstream sensor element is warmed by heat dissipated in the air stream.
The problem with traditional thermal dissipation methods of the type described above is that it is not possible to distinguish between large changes in ambient temperature and changes in air flow. One method that is often useful in handling variations in ambient temperature is to use a reference sensor that is not exposed to the air stream as a baseline for comparison. In this method, the sensor elements are heated to some temperature above ambient (it is assumed that the air stream being detected will be at ambient temperature and will provide a cooling effect). This method works well for compensating for changes in ambient conditions. However, a problem with this method exists that is specific to the HVAC accessory application. The problem lies in the fact the temperature of the air stream being detected is often different from ambient. This condition would be difficult of an application where the HVAC accessory equipment is installed in an unconditioned space (attic or garage). For this type of installation, the accessory equipment is exposed to widely varying ambient temperatures while the air and the duct is near the temperature of the conditioned living space. For very cold ambient conditions, it is not sufficient to heat the sensor elements to just any fixed differential above ambient because the air stream will be likely even warmer yet. For the case where the air stream being detected is warmer than the heated sensing element, the predicted cooling effect will not exist.
Accordingly, it would be desirable to provide an improved device for detecting air flow in HVAC applications over a wide range of temperatures which overcomes the problems discussed above.
It is a general object of the present invention to provide air detection apparatus for sensing air flow in HVAC systems so as to control the operation of accessory items mounted therein.
It is one object of the present invention to provide an air detection apparatus which will function efficiently even though the temperature of an air stream being detected is different from the ambient temperature.
It is an additional object of the present invention to provide an air detection apparatus which heats sensor elements to a target temperature greater than the highest specified operating ambient temperature.
It is a further object of the present invention to provide an air detection Up apparatus which uses a combination of self-heating and external heating of sensor elements in order to compensate for wide variations in ambient temperature.
It is also an object of the present invention to provide an air detection apparatus which is easier to install requiring less wiring and no interface with a blower motor and which can be installed in a door assembly on the accessory item.
In one aspect of the invention, an air flow detection apparatus is provided which is useful in operating an accessory item in an HVAC system. The apparatus includes a sensor printed circuit board assembly having a first thermistor adapted to be exposed to an air flow associated with a first pair of heating resistors, and a second thermistor in parallel with the first thermistor adapted to be hidden from the air flow and associated with a second pair of heating resistors. A control printed circuit board assembly is interconnected with the sensor printed circuit board assembly and has a voltage regulator circuit for applying a DC voltage source to a power regulator circuit and a sensor monitoring circuit interfaced therewith. The power regulator circuit enables the first thermistor and the second thermistor to be heated to a target temperature greater than a highest specified operating ambient temperature and maintains the temperature of the second thermistor regardless of the actual ambient temperature. The sensor monitoring circuit responds to the cooling of the first thermistor in the presence of air flow to output a signal operating the HVAC accessory item. The power regulator circuit includes a differential amplifier outputting through a first transistor to control the temperatures of the first thermistor and the second thermistor. The sensor monitoring circuit includes an operational amplifier outputting to a second transistor to sense the difference in temperature between the first thermistor and the second thermistor in the presence of air flow. The power regulator circuit applies a common voltage across a first branch defined by the first thermistor and the first pair of heating resistors, and a second branch defined by the second thermistor and the second pair of heating resistors. The first pair of heating resistors are in parallel and the second pair of heating resistors are in parallel.
In another aspect of the invention, an air flow detection apparatus for operating an accessory item in an HVAC system includes a sensor circuit board assembly and a control circuit board assembly interconnected with the sensor circuit board assembly and having a voltage regulator circuit for supplying a voltage source to a power regulator circuit and a sensor monitoring circuit interfaced therewith. The sensor circuit board assembly has a first branch including a first thermistor connected with a pair of parallel heating resistors, and a second branch including a second thermistor connected with a second pair of heating resistors. The sensor circuit board assembly is enclosed in a sensor housing having a first chamber exposed to an air flow for holding the first branch, and a second chamber hidden from the air flow for holding the second branch. The sensor housing is adapted to be mounted on the accessory item in the HVAC system. A gasket is sandwiched between the sensor circuit board assembly and the sensor housing. The gasket is formed with a first opening through which the first branch projects into the first chamber, and a second opening through which the second branch projects into the second chamber. A bottom wall of the sensor housing is formed with a pair of spaced apart ports communicating the first branch with an air flow. The control circuit board assembly includes a transformer and a network of diodes for full wave rectifying a source of AC voltage into a stepped down DC voltage.
In yet another aspect of the invention, a method of detecting air flow so as to control the operation of an accessory item in an HVAC system includes the steps of disposing a first thermistor and a pair of heating resistors in parallel with a second thermistor and a second pair of heating resistors; exposing the first thermistor and a first pair of heating resistors to an air flow and isolating the second thermistor and second pair of heating resistors from the air flow; providing a power regulator circuit to heat the first thermistor and second thermistor to a target temperature greater than a highest specified operating ambient temperature and maintain the temperature of the second thermistor regardless of varying ambient temperature; and interconnecting the power regulating circuit with a sensor monitoring circuit for sensing the difference in temperature of the first thermistor and the second thermistor in the presence of air flow to output a signal operating the HVAC accessory item.
Various other objects, features and advantages of the invention will be made apparent from the following description taken together with the drawings.